Pupil-replicating lightguide with switchable out-coupling efficiency distribution and display based thereon

ABSTRACT

A pupil-replicating lightguide includes a slab of transparent material for guiding image light in the slab, and an out-coupling structure supported by the slab for out-coupling portions of the image light from the slab. The portions are laterally offset from one another along a path of the image light in the slab. The out-coupling grating structure has a switchable distribution of out-coupling efficiency for redirecting the portions of out-coupled light to a desired location such as a current location of an eye of the viewer determined by an eye tracking system. The out-coupling grating structure may include a plurality of diffraction gratings having different local slant angles of grating fringes.

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Pat. App. No. 63/286,381 entitled “Display Applications of Switchable Gratings”, and U.S. Provisional Pat. App. No. 63/286,230 entitled “Active Fluidic Optical Element”, both filed on Dec. 6, 2021 and incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to tunable optical devices, and in particular to lightguides usable in visual displays, as well as components, modules, and methods for lightguides and visual displays.

BACKGROUND

Visual displays provide information to viewer(s) including still images, video, data, etc. Visual displays have applications in diverse fields including entertainment, education, engineering, science, professional training, advertising, to name just a few examples. Some visual displays such as TV sets display images to several users, and some visual display systems such s near-eye displays (NEDs) are intended for individual users.

An artificial reality system generally includes an NED (e.g., a headset or a pair of glasses) configured to present content to a user. The near-eye display may display virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view images of virtual objects (e.g., computer-generated images (CGIs)) superimposed with the surrounding environment by seeing through a “combiner” component. The combiner component including its light routing optics may be transparent to external light.

An NED is usually worn on the head of a user. Consequently, a large, bulky, unbalanced, and heavy display device with a heavy battery would be cumbersome and uncomfortable for the user to wear. Head-mounted display devices can benefit from a compact and energy-efficient configuration, including efficient light sources and illuminators providing illumination of a display panel, high-throughput combiner components and ocular lenses, and other optical elements in the image forming train that can provide an image to a user’s eye with minimal image distortions and artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with the drawings, in which:

FIG. 1 is a schematic side view of a pupil-replicating lightguide with switchable out-coupling efficiency distribution;

FIG. 2 is a schematic side view of an embodiment of an out-coupling grating structure of the pupil-replicating lightguide of FIG. 1 , the out-coupling grating structure having a plurality of switchable diffraction gratings;

FIG. 3 is a schematic side view of a near-eye display including a pupil-replicating lightguide of FIG. 1 ;

FIG. 4 is a flow chart of a method for displaying an image;

FIG. 5 is side cross-sectional views of a tunable liquid crystal (LC) surface-relief grating usable in a pupil-replicating lightguide of this disclosure;

FIG. 6A is a side cross-sectional view of a polarization volumetric grating (PVH) usable in a pupil-replicating lightguide of this disclosure;

FIG. 6B is a diagram illustrating optical performance of the PVH of FIG. 6A;

FIG. 7A is a side cross-sectional view of a fluidic grating usable in a pupil-replicating lightguide of this disclosure, in an OFF state;

FIG. 7B is a side cross-sectional view of the fluidic grating of FIG. 7A in an ON state;

FIG. 8 is a side cross-sectional view of a pair of wavelength-selective Bragg gratings usable in a pupil-replicating lightguide of this disclosure;

FIG. 9 is a plan view of a near-eye display of this disclosure having a form factor of a pair of eyeglasses; and

FIG. 10 is a three-dimensional view of a head-mounted display (HMD) of this disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated.

Near-eye displays may use pupil-replicating lightguides to carry images to user’s eyes. Pupil-replicating lightguides have an advantage of not requiring ocular lenses for viewing the image. Pupil-replicating lightguides can transmit the outside light without refocusing or redirection, allowing their use as combiner elements in augmented reality (AR) display systems.

Pupil-replicating lightguides may have a drawback of low optical throughput, in part due to having to spread the input image light beam over a large area of an eyebox of the display. Since a user’s eye, more precisely user’s eye pupil, occupies only a small portion of a total eyebox area, most of the light out-coupled by a pupil-replicating lightguide is lost. Most of the broadly spread light does not enter the eye pupil, instead illuminating outer areas of the user’s eyes and user’s face. Another source of optical throughput loss is related to having to optimize the out-coupling grating of the pupil-replicating lightguide to operate in a uniform manner over the entire eyebox. Such parameters as grating strength, grating fringes slant angles, etc., need to be optimized to account for losses of optical power as the image light propagates through the waveguides while being out-coupled, for the purpose of providing a uniform distribution of the image light over the entire eyebox area. The optimization of grating parameters for uniformity may result in a drop of overall optical throughput.

In accordance with this disclosure, an overall effective throughput of a pupil-replicating lightguide may be increased by providing an out-coupling grating structure having a switchable distribution of out-coupling efficiency. The switchable out-coupling efficiency may be provided by a grating having structure a variable or switchable slant angle of the grating fringes. The slant angle of the grating fringes that out-couple the image light may be varied to direct more light energy towards a particular location of the eye pupil. The current eye pupil location may be determined by an eye tracking system.

In some embodiments, a switchable out-coupling grating structure may include a plurality of diffraction gratings having different local slant angles of grating fringes. The diffraction gratings of the grating structure may be made switchable between a high-efficiency state where a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state where a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold, e.g. 10 times or even 100 times lower.

The gratings may be physically switched ON and OFF. In some embodiments, the gratings may have a wavelength-selective refractive index contrast, and the switching may be achieved by quickly switching the illuminating wavelength causing one grating to diffract the light being guided by the pupil-replicating lightguide and another grating to be transparent to such light.

In accordance with the present disclosure, there is provided a pupil-replicating lightguide for expanding image light. The pupil-replicating lightguide includes a slab of transparent material for guiding the image light in the slab by a series of internal reflections from opposed surfaces of the slab, and an out-coupling grating structure supported by the slab for out-coupling portions of the image light from the slab. The portions are laterally offset from one another along a path of the image light in the slab. The out-coupling grating structure has a switchable distribution of out-coupling efficiency.

The out-coupling grating structure may include a plurality of diffraction gratings having different local slant angles of grating fringes. The diffraction gratings are switchable between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. The second threshold may be e.g. at least 10 times, or at least 100 lower than the first threshold. The plurality of switchable diffraction gratings may include switchable gratings with spatially varying slant angle of the grating fringes along the path of the image light. The switchable gratings may be disposed in a stack configuration parallel to the opposed surfaces of the slab. The plurality of diffraction gratings may include a polarization volume hologram (PVH) grating, a tunable liquid crystal (LC) surface-relief grating, a fluidic grating, etc. The out-coupling grating structure may include a grating having a wavelength-dependent refractive index contrast.

In accordance with the present disclosure, there is provided a near-eye display comprising a projector for providing image light, and an above described pupil-replicating lightguide coupled to the projector. The near-eye display may further include an eye tracker for determining a position of a pupil of a user’s eye at an eyebox of the near-eye display, and a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to cause the eye tracker to determine the position of the pupil, and, responsive to the determined position of the pupil, switch the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position.

In some embodiments, the near-eye display may further include an eye tracker for determining a position of a pupil of the user’s eye at an eyebox of the near-eye display, and a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to cause the eye tracker to determine the position of the pupil; and, responsive to the determined position of the pupil, switch a diffraction grating of the plurality of diffraction gratings to the high-efficiency state. The controller may be further configured to switch the remaining diffraction gratings of the plurality of diffraction gratings to the low-efficiency state.

In accordance with the present disclosure, there is further provided a method for displaying an image. The method includes providing image light to a pupil-replicating lightguide comprising a slab of transparent material, guiding the image light in the slab by a series of internal reflections from opposed surfaces of a slab of transparent material, out-coupling portions of the image light from the slab by an out-coupling grating structure, wherein the portions are laterally offset from one another along a path of the image light in the slab, and switching angular distribution of out-coupling efficiency of a plurality of grating fringes of the out-coupling grating structure. The switching of the angular distribution of the out-coupling efficiency may include switching a plurality of diffraction gratings having different local slant angles of grating fringes, where the diffraction gratings are switched between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. The method may further include using an eye tracker to determine a position of a pupil of a user’s eye at an eyebox of the near-eye display; and, responsive to the determined position of the pupil, switching the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position.

Referring now to FIG. 1 , a pupil-replicating lightguide 100 includes a slab 102 of optically transparent material, i.e. transparent in visible wavelength range, configured for guiding image light 104 in the slab 102 by a series of internal reflections from opposed surfaces 111, 112 of the slab 102. An in-coupler 106 supported by the slab 102 may be provided to in-couple the image light 104 into the slab 102. An out-coupling grating structure 108 supported by the slab 102 may out-couple portions 114 of the image light 104 from the slab 102. The portions 114 of the image light 104 may be laterally offset from one another along a path 116 of the image light 104 in the slab 102. The path 116 is denoted with dashed lines. The slab 102 is parallel to the XY plane in FIG. 1 , and the image light 104 propagates in XY plane, so that the portions 114 are offset along both X- and Y-directions in FIG. 1 . The pupil-replicating lightguide 100 expands the image light in XY plane. Only several possible locations of the portions 114 are shown for brevity.

The out-coupling grating structure 108 has a switchable or tunable distribution of out-coupling efficiency, enabling the control of locations and/or distribution of angles of out-coupling the image light portions 114 with high efficiency at the account of other locations and angles where the image light portions 114 are out-coupled with a lesser efficiency. In other words, the distribution of the out-coupling efficiency in XY plane and/or out-coupling angle direction may be tuned or switched. Herein and throughout the rest of the specification, the terms “tunable” and “switchable” are used interchangeably.

For example, at a first location 121, the image light portions 114 can be out-coupled with maximum light energy converging at locations A, B, or C, as required. At a second location 122, the image light portions 114 can be out-coupled with maximum light energy converging at locations D, E, or F, as needed. It is further noted that, while the out-coupling angles depend on the in-coupling angles and the grating pitch of the in-coupler 106 and the out-coupling gratings structure 108, the maximum flow of energy in a particular direction or location on the out-coupling grating structure 108 depends on refractive index contrast and the local grating slant angle.

Referring to FIG. 2 , an out-coupling grating structure 208 is an embodiment of the out-coupling grating structure 108 of the pupil-replicating lightguide 100 of FIG. 1 . The out-coupling grating structure 208 of FIG. 2 includes first 201 and second 202 diffraction gratings having different local slant angles of grating fringes 211 and 212, respectively. In the embodiment shown, the first 201 and second 202 diffraction gratings have a spatially varying slant angle of the grating fringes 211 and 212, respectively, along Y-direction, going from more parallel to XY plane to more orthogonal to the XY plane from left to right in the non-limiting example of FIG. 2 . In other words, the angle between the planes of the fringes and Z axis increases in going from left to right. The slant angle of the grating fringes 211 and 212 may vary along the path 116 of the image light 104 in the slab 102. The first 201 and second 202 diffraction gratings may be disposed in a stack configuration as shown, parallel to the opposed surfaces 111, 112.

The diffraction gratings 201 and 202 of FIG. 2 are switchable between a high-efficiency or ON state, in which a percentage of the image light 104 out-coupled from the slab 102 is above a first threshold, and a low-efficiency or OFF state, in which a percentage of the image light 104 out-coupled from the slab 102 is below a second threshold lower than the first threshold. The second threshold may be e.g. at least ten times lower than the first threshold, or in some embodiments, at least one hundred times lower than the first threshold. When the first diffraction grating 201 is in the ON state while the second diffraction grating 202 is in the OFF state, a major portion of the image light 104 propagating along the path 116 is out-coupled as a first portion 214 at a first location 221. The slant angle of the fringes of the first diffraction grating 201 is spatially varies along the Y-axis, so as to redirect, concentrate, or converge the first portion 214 at the first location 221, providing a better illumination of the first location 221. When the first diffraction grating 201 is in the OFF state while the second diffraction grating 202 is in the ON state, a major portion of the image light 104 propagating along the path 116 is out-coupled as a second portion 215 converging at a second location 222. The slant angle variation of the fringes 211 and 212 facilitates the convergence of the first 214 and second 215 portions onto the first 221 and second 222 locations, respectively. More than two switchable diffraction gratings 201 and 202 may be provided in the out-coupling grating structure 208, and may be disposed in a stack configuration. The controller 368 may be configured to switch at least one of the diffraction gratings to the ON state while, optionally, switching the remaining diffraction gratings of the stack to the OFF state.

Turning to FIG. 3 with further reference to FIGS. 1 and 2 , a near-eye display 350 includes a projector 352 coupled to the pupil-replicating lightguide 100 of FIG. 1 . The projector 352 of FIG. 3 includes a microdisplay panel 354 coupled to a collimator 356 e.g. a lens and/or a reflective element having optical power, for converting an image in linear domain into an image in angular domain. Herein, the term “image in angular domain” means an image where which different elements of an image in linear or spatial domain, i.e. pixels of the image displayed by the microdisplay panel 354, are represented by angles of corresponding rays of the image light 104 downstream of the collimator 356, the rays carrying optical power levels and/or color composition corresponding to brightness and/or color values of the image pixels. A scanning projector may be used in place of the microdisplay-based projector 352. A scanning projector may scan or raster an image in angular domain pixel by pixel, or in group of pixels.

The pupil-replicating lightguide 100 carries the image light 304 generated by the microdisplay 354 to an eyebox 360 by delivering portions of the image light 304 to the eyebox 360, enabling a user’s eye 362 to directly view the image. The image can be viewed in superposition with, or overlaid upon, a view of external environment of the user, because external light 364 can propagate through the pupil-replicating lightguide 100 without being refocused or redirected. Thus, the pupil-replicating lightguide 100 functions as an effective combiner element superimposing the display-generated imagery with views of surrounding real-life environment.

The near-eye display 350 may utilize an eye tracker 366 for determining a position of a pupil of the user’s eye 362 at the eyebox 360 of the near-eye display 350, and a controller 368 operably coupled to the projector 352, the eye tracker 366, and the pupil-replicating lightguide 100. The controller 368 may be suitably configured, e.g. programmed via software and/or firmware, hardwired, etc., to cause the eye tracker 366 to determine the position of the eye pupil. Responsive to the determined position of the eye pupil, the controller 368 may switch the angular distribution of diffraction efficiency of the out-coupling grating structure 108 as explained above with reference to FIGS. 1 and 2 , to increase an amount of the image light 304 illuminating the pupil at the determined position.

For example, in embodiments where the pupil-replicating lightguide 100 of FIGS. 1 and 3 includes the out-coupling grating structure 208 of FIG. 2 having the first 201 and second 202 switchable diffraction gratings, the controller 368 may be configured to cause the eye tracker 366 to determine the position of the pupil of the eye 362 to be a first position 321. Responsive to the determined position of the eye pupil to be at the first location 321, the controller 368 may switch the first diffraction grating 201 to the high-efficiency state while optionally switching the second diffraction grating 202 to the low-efficiency state. This causes a first out-coupled light portion 314 of the image light 304 to be directed at the first location 321. When the eye tracker 366 determines that the eye 362 is at a second position 322, the controller 368 may switch the first diffraction grating 201 to the low-efficiency state while switching the second diffraction grating 202 to the high-efficiency state. This causes a second out-coupled light portion 315 of the image light 304 to be directed at the second location 322 while suppressing the first out-coupled light portion 314 and thus preserving energy. When more than two switchable diffraction gratings are provided in the out-coupling grating structure 208, the out-coupled light portions may be directed to more locations at the eyebox 360.

Referring now to FIG. 4 with further reference to FIGS. 2 and 3 , a method 400 for displaying an image includes providing (402) image light to a pupil-replicating lightguide. For example, the image light 304 may be provided by the projector 352 to the pupil-replicating lightguide 100 for propagation in the slab 102. The image light 304 is guided (404) in the slab 102 by a series of internal reflections from opposed surfaces of the slab 102. Portions of the image light 304 are out-coupled (406) from the slab 102 by an out-coupling grating structure, e.g. the out-coupling grating structure 208 of FIG. 2 . The portions may be laterally offset from one another along a path of the image light 304 in the slab 102. Angular distribution of out-coupling efficiency of a plurality of grating fringes of the out-coupling grating structure is switched (408) to direct the image light portions to a desired location.

As explained above with reference to FIGS. 2 and 3 , the switching of the angular distribution of the out-coupling efficiency may include switching a plurality of diffraction gratings having different local slant angles of grating fringes, e.g. the first 201 and second 202 diffraction gratings of the out-coupling grating structure 208 of FIG. 2 . The first 201 and second 202 diffraction gratings are switched between the high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and the low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. The method 400 of FIG. 4 may further include using an eye tracker, e.g. the eye tracker 366 of FIG. 3 , to determine (410) a position of a pupil of the user’s eye 362 at an eyebox 360 of the near-eye display 350. Responsive to the determined position of the pupil, the angular distribution of diffraction efficiency may be switched (412) to increase an amount of the image light illuminating the pupil at the determined position.

Non-limiting illustrative examples of switchable diffraction gratings usable in lightguides and displays of this disclosure will now be presented.

Referring first to FIG. 5 , a tunable liquid crystal (LC) surface-relief grating 500 may be used as a switchable diffraction grating of the out-coupling grating structure 208 of FIG. 2 . The tunable LC surface-relief grating 500 includes a first substrate 501 supporting a first conductive layer 511 and a surface-relief grating structure 504 having a plurality of ridges 506 extending from the first substrate 501 and/or the first conductive layer 511.

A second substrate 502 is spaced apart from the first substrate 501. The second substrate 502 supports a second conductive layer 512. A cell is formed by the first 511 and second 512 conductive layers. The cell is filled with a LC fluid, forming an LC layer 508. The LC layer 508 includes nematic LC molecules 510, which may be oriented by an electric field across the LC layer 508. The electric field may be provided by applying a voltage V to the first 511 and second 512 conductive layers.

The surface-relief grating structure 504 may be polymer-based, e.g. it may be formed from a polymer having an isotropic refractive index n_(p) of about 1.5, for example. The LC fluid has an anisotropic refractive index. For light polarization parallel to a director of the LC fluid, i.e. to the direction of orientation of the nematic LC molecules 510, the LC fluid has an extraordinary refractive index n_(e), which may be higher than an ordinary refractive index n_(o) of the LC fluid for light polarization perpendicular to the director. For example, the extraordinary refractive index n_(e) may be about 1.7, and the ordinary refractive index n_(o) may be about 1.5, i.e. matched to the refractive index n_(p) of the surface-relief grating structure 504.

When the voltage V is not applied (left side of FIG. 5 ), the LC molecules 510 are aligned approximately parallel to the grooves of the surface-relief grating structure 504. At this configuration, a linearly polarized light beam 521 with e-vector oriented along the grooves of the surface-relief grating structure 504 will undergo diffraction, since the surface-relief grating structure 504 will have a non-zero refractive index contrast. When the voltage V is applied (right side of FIG. 5 ), the LC molecules 510 are aligned approximately perpendicular to the grooves of the surface-relief grating structure 504. At this configuration, a linearly polarized light beam 521 with e-vector oriented along the grooves of the surface-relief grating structure 504 will not undergo diffraction because the surface-relief grating structure 504 will appear to be index-matched and, accordingly, will have a substantially zero refractive index contrast. For the linearly polarized light beam 521 with e-vector oriented perpendicular to the grooves of the surface-relief grating structure 504, no diffraction will occur in either case (i.e. when the voltage is applied and when it is not) because at this polarization of the linearly polarized light beam 521, the surface-relief grating structure 504 are index-matched. Thus, the tunable LC surface-relief grating 500 can be switched on and off (for polarized light) by controlling the voltage across the LC layer 508. Several such gratings with differing pitch / slant angle / refractive index contrast may be used to switch between several grating configurations.

In some embodiments of the LC surface-relief grating 500, the surface-relief grating structure 504 may be formed from an anisotropic polymer with substantially the same or similar ordinary n_(o) and extraordinary n_(e) refractive indices as the LC fluid itself. When the LC director aligns with the optic axis of the birefringent polymer, the refractive index contrast is close to zero at any polarization of impinging light, and there is no diffraction. When the LC director is misaligned with the optic axis of the birefringent polymer e.g. due to application of an external electric field, the refractive index contrast is non-zero for any or most polarizations of the impinging light, and accordingly there is diffraction and beam deflection.

Turning to FIG. 6A, a polarization volume hologram (PVH) grating 600 may be used as a switchable diffraction grating of the out-coupling grating structure 208 of FIG. 2 . The PVH grating 600 of FIG. 6A includes an LC layer 604 bound by opposed top 605 and bottom 606 parallel surfaces. The LC layer 604 may include an LC fluid containing rod-like LC molecules 607 with positive dielectric anisotropy, i.e. nematic LC molecules. A chiral dopant may be added to the LC fluid, causing the LC molecules in the LC fluid to self-organize into a periodic helical configuration including helical structures 608 extending between the top 605 and bottom 606 parallel surfaces of the LC layer 604. Such a configuration of the LC molecules 607, termed herein a cholesteric configuration, includes a plurality of helical periods p, e.g. at least two, at least five, at least ten, at least twenty, or at least fifty helical periods p between the top 605 and bottom 606 parallel surfaces of the LC layer 604.

Boundary LC molecules 607 b at the top surface 605 of the LC layer 604 may be oriented at an angle to the top surface 605. The boundary LC molecules 607 b may have a spatially varying azimuthal angle, e.g. linearly varying along X-axis parallel to the top surface 605, as shown in FIG. 6A. To that end, an alignment layer 612 may be provided at the top surface 605 of the LC layer 604. The alignment layer 612 may be configured to provide the desired orientation pattern of the boundary LC molecules 607 b, such as the linear dependence of the azimuthal angle on the X-coordinate. A pattern of spatially varying polarization directions of the UV light may be selected to match a desired orientation pattern of the boundary LC molecules 607 b at the top surface 605 and/or the bottom surface 606 of the LC layer 604. When the alignment layer 612 is coated with the cholesteric LC fluid, the boundary LC molecules 607 b are oriented along the photopolymerized chains of the alignment layer 612, thus adopting the desired surface orientation pattern. Adjacent LC molecules adopt helical patterns extending from the top 605 to the bottom 606 surfaces of the LC layer 604, as shown.

The boundary LC molecules 607 b define relative phases of the helical structures 608 having the helical period p. The helical structures 608 form a volume grating comprising helical fringes 614 tilted at an angle ϕ, as shown in FIG. 6A. The steepness of the tilt angle ϕ depends on the rate of variation of the azimuthal angle of the boundary LC molecules 607 b at the top surface 605 and p. Thus, the tilt angle ϕ is determined by the surface alignment pattern of the boundary LC molecules 607 b at the alignment layer 612. The volume grating has a period Λ_(x) along X-axis and Λ_(y) along Y-axis. In some embodiments, the periodic helical structures 608 of the LC molecules 607 may be polymer-stabilized by mixing in a stabilizing polymer into the LC fluid, and curing (polymerizing) the stabilizing polymer.

The helical nature of the fringes 614 of the volume grating makes the PVH grating 600 preferably responsive to light of polarization having one particular handedness, e.g. left- or right- circular polarization, while being substantially non-responsive to light of the opposite handedness of polarization. Thus, the helical fringes 614 make the PVH grating 600 polarization-selective, causing the PVH grating 600 to diffract light of only one handedness of circular polarization. This is illustrated in FIG. 6B, which shows a light beam 620 impinging onto the PVH grating 600. The light beam 620 includes a left circular polarized (LCP) beam component 621 and a right circular polarized (RCP) beam component 622. The LCP beam component 621 propagates through the PVH grating 600 substantially without diffraction. Herein, the term “substantially without diffraction” means that, even though an insignificant portion of the beam (the LCP beam component 621 in this case) might diffract, the portion of the diffracted light energy is so small that it does not impact the intended performance of the PVH grating 600. The RCP beam component 622 of the light beam 620 undergoes diffraction, producing a diffracted beam 622′. The polarization selectivity of the PVH grating 600 results from the effective refractive index of the grating being dependent on the relationship between the handedness, or chirality, of the impinging light beam and the handedness, or chirality, of the grating fringes 614. Changing the handedness of the impinging light may be used to switch the performance of the PVH grating 600. The PVH grating 600 may also be made tunable by applying voltage to the LC layer 604, which distorts or erases the above-described helical structure. It is further noted that sensitivity of the PVH 600 to right circular polarized light in particular is only meant as an illustrative example. When the handedness of the helical fringes 614 is reversed, the PVH 600 may be made sensitive to left circular polarized light. Thus, the operation of the PVH 600 may be controlled by controlling the polarization state of the impinging light beam 620. The PVH 600 may be made tunable by application of electric field across the LC layer 604, which erases the periodic helical structures 608.

Referring now to FIGS. 7A and 7B, a fluidic grating 700 may be used as a switchable diffraction grating of the out-coupling grating structure 208 of FIG. 2 . The fluidic grating 700 includes first 701 and second 702 immiscible fluids separated by an inter-fluid boundary 703. One of the first 701 and second 702 fluids may be a hydrophobic fluid such as oil, e.g. silicone oil, while the other fluid may be water-based. One of the first 701 and second 702 fluids may be a gas in some embodiments. The first 701 and second 702 fluids may be contained in a cell formed by first 711 and second 712 substrates supporting first 721 and second 722 electrode structures. The first 721 and/or second 722 electrode structures may be at least partially transparent, absorptive, and/or reflective.

At least one of the first 721 and second 722 electrode structures may be patterned for imposing a spatially variant electric field onto the first 701 and second 702 fluids. For example, in FIGS. 7A and 7B, the first electrode 721 is patterned, and the second electrodes 722 is not patterned, i.e. the second electrodes 722 is a backplane electrode. In the embodiment shown, both the first 721 and second 722 electrodes are substantially transparent. For example, the first 721 and second 722 electrodes may be indium tin oxide (ITO) electrodes.

FIG. 7A shows the fluidic grating 700 in a non-driven state when no electric field is applied across the inter-fluid boundary 703. When no electric field is present, the inter-fluid boundary 703 is straight and smooth; accordingly, a light beam 705 impinging onto the fluidic grating 700 does not diffract, propagating right through as illustrated. FIG. 7B shows the fluidic grating 700 in a driven state when a voltage V is applied between the first 721 and second 722 electrodes, producing a spatially variant electric field across the first 701 and second 702 fluids separated by the inter-fluid boundary 703.

The application of the spatially variant electric field causes the inter-fluid boundary 703 to distort as illustrated in FIG. 7B, forming a periodic variation of effective refractive index, i.e. a surface-relief diffraction grating. The light beam 705 impinging onto the fluidic grating 700 will diffract, forming first 731 and second 732 diffracted sub-beams. By varying the amplitude of the applied voltage V, the strength of the fluidic grating 700 may be varied. By applying different patterns of the electric field e.g. with individually addressable sub-electrodes or pixels of the first electrode 721, the grating period and, accordingly, the diffraction angle, may be varied. More generally, varying the effective voltage between separate sub-electrodes or pixels of the first electrode 721 may result in a three-dimensional conformal change of the fluidic interface i.e. the inter-fluid boundary 703 inside the fluidic volume to impart a desired optical response to the fluidic grating 700, including grating pitch, slant angle, etc. The applied voltage pattern may be pre-biased to compensate or offset gravity effects, i.e. gravity-caused distortions of the inter-fluid boundary 703.

Portions of a patterned electrode may be individually addressable. In some embodiments, the patterned electrode 721 may be replaced with a continuous, non-patterned electrode coupled to a patterned dielectric layer for creating a spatially nonuniform electric field across the first 701 and second 702 fluids. Also in some embodiments, the backplane electrode is omitted, and the voltage is applied between the segmented electrodes themselves.

The thickness of the first 721 and second 722 electrodes may be e.g. between 10 nm and 50 nm. The materials of the first 721 and second 722 electrodes besides ITO may be e.g. indium zinc oxide (IZO), zinc oxide (ZO), indium oxide (IO), tin oxide (TO), indium gallium zinc oxide (IGZO), silver nanowires, carbon nanotubes, indium tin oxide (ITO), ITO/Ag/ITO trilayer film, etc. The first 701 and second 702 fluids may have a refractive index difference of at least 0.1, and may be as high as 0.2 and higher. One of the first 701 or second 702 fluids may include polyphenylether, 1,3-bis(phenylthio)benzene, etc. The first 711 and/or second 712 substrates may include e.g. fused silica, quartz, sapphire, etc. The first 711 and/or second 712 substrates may be straight or curved, and may include vias and other electrical interconnects. The applied voltage may be varied in amplitude and/or duty cycle when applied at a frequency of between 100 Hz and 100 kHz. The applied voltage can change polarity and/or be bipolar. Individual first 701 and/r second 702 fluid layers may have a thickness of between 0.5-5 micrometers, more preferably between 0.5-2 micrometer.

To separate the first 701 and second 702 fluids, surfactants containing one hydrophilic end functional group and one hydrophobic end functional group may be used. The examples of a hydrophilic end functional group are hydroxyl, carboxyl, carbonyl, amino, phosphate, sulfhydryl. The hydrophilic functional groups may also be anionic groups such as sulfate, sulfonate, carboxylates, phosphates, for example. Non-limiting examples of a hydrophobic end functional group are aliphatic groups, aromatic groups, fluorinated groups. For example, when polyphenyl thioether and fluorinated fluid may be selected as a fluid pair, a surfactant containing aromatic end group and fluronirated end group may be used. When phenyl silicone oil and water are selected as the fluid pair, a surfactant containing aromatic end group and hydroxyl (or amino, or ionic) end group may be used. These are only non-limiting examples.

Referring to FIG. 8 , a wavelength-selective grating structure 800 is a variant of the out-coupling grating structure 208 of FIG. 2 . The wavelength-selective grating structure 800 comprises a slab 802 of transparent material, a first volume Bragg grating (VBG) 804, and a second VBG 805, which may be at different pitch (spatial period) and/or different slant angle as illustrated. The first 804 and second 805 VBGs are wavelength-selective: the first VBG 804 diffracts light at a first wavelength while not diffracting light at a second, different wavelength, while the second VBG 805 diffracts light at the second wavelength while not diffracting light at the first wavelength. The two wavelengths may belong to a same color channel, and a quickly tunable or switchable laser, with a switching time of the emission wavelength on the order of microseconds, may be used to select between the first 804 and second 805 VBGs. The pair of VBGs 804, 805 has a performance similar to that of a single VBG switchable in pitch and/or slant angle. The wavelength selectivity of the VBGs 804, 805 may be provided e.g. by a wavelength-dependent refractive index contrast. The refractive index contrast is defined herein as a difference between the refractive index of the fringes and the refractive index of the slab 802 supporting the VBG fringes.

Some switchable gratings include a material with tunable refractive index. By way of a non-limiting example, a holographic polymer-dispersed liquid crystal (H-PDLC) grating may be manufactured by causing interference between two coherent laser beams in a photosensitive monomer/liquid crystal (LC) mixture contained between two substrates coated with a conductive layer. Upon irradiation, a photoinitiator contained within the mixture initiates a free-radical reaction, causing the monomer to polymerize. As the polymer network grows, the mixture phase separates into polymer-rich and liquid-crystal rich regions. The refractive index modulation between the two phases causes light passing through the cell to be scattered in the case of traditional PDLC or diffracted in the case of H-PDLC. When an electric field is applied across the cell, the index modulation is removed and light passing through the cell is unaffected. This is described in an article entitled “Electrically Switchable Bragg Gratings from Liquid Crystal/Polymer Composites” by Pogue et al., Applied Spectroscopy, v. 54 No. 1, 2000, which is incorporated herein by reference in its entirety.

Tunable or switchable gratings with a variable grating period may be produced e.g. by using flexoelectric LC. For LCs with a non-zero flexoelectric coefficient difference (e1-e3) and low dielectric anisotropy, electric fields exceeding certain threshold values result in transitions from the homogeneous planar state to a spatially periodic one. Field-induced grating is characterized by rotation of the LC director about the alignment axis with the wavevector of the grating oriented perpendicular to the initial alignment direction. The rotation sign is defined by both the electric field vector and the sign of the (e1-e3) difference. The wavenumber characterizing the field-induced periodicity is increased linearly with the applied voltage starting from a threshold value of about π/d, where d is the thickness of the layer. A description of flexoelectric LC gratings may be found e.g. in an article entitled “Dynamic and Photonic Properties of Field-Induced Gratings in Flexoelectric LC Layers” by Palto in Crystals 2021, 11, 894, which is incorporated herein by reference in its entirety.

Tunable gratings with a variable grating period or a slant angle may be provided e.g. by using helicoidal LC. Tunable gratings with helicoidal LCs have been described e.g. in an article entitled “Electrooptic Response of Chiral Nematic Liquid Crystals with Oblique Helicoidal Director” by Xiang et al. Phys. Rev. Lett. 112, 217801, 2014, which is incorporated herein by reference in its entirety.

For gratings exhibiting strong wavelength dependence of grating efficiency, several gratings, e.g. several volumetric Bragg grating (VBG) gratings, may be provided in the lightguide. The gratings that diffract light at any given moment of time may be switched by switching the VBG grating on and off, and/or by switching the wavelength of the light propagating in the waveguide.

Referring now to FIG. 9 , a near-eye display (NED) 900 is one possible implementation of a display apparatus of this disclosure. The NED 900 includes a frame 901 supporting, for each eye: a projector 908; a pupil-replicating lightguide 910 for guiding the illuminating light beam inside and out-coupling portions of the illuminating light beam as disclosed herein; an eye tracker 904 for obtaining images of a user’s eye in an eyebox 912; and a plurality of eyebox illuminators 906 shown as black dots. The eyebox illuminators 906 may be supported by the pupil-replicating lightguide 910 for illuminating the eyebox 912.

The purpose of the eye trackers 904 is to determine position and/or orientation of both eyes of the user to enable steering the output image light to the locations of the user’s eyes as disclosed herein. The eyebox illuminators 906 illuminate the eyes at the corresponding eyeboxes 912, to enable the eye trackers 904 to obtain the images of the eyes, as well as to provide reference reflections i.e. glints. The glints may function as reference points in the captured eye image, facilitating the eye gazing direction determination by determining position of the eye pupil images relative to the glints images. To avoid distracting the user with the light of the eyebox illuminators 906, the light illuminating the eyeboxes 912 may be made invisible to the user. For example, infrared light may be used to illuminate the eyeboxes 912.

Turning to FIG. 10 , an HMD 1000 is an example of an AR/VR wearable display system which encloses the user’s face, for a greater degree of immersion into the AR/VR environment. The HMD 1000 may generate the entirely virtual 3D imagery. The HMD 1000 may include a front body 1002 and a band 1004 that can be secured around the user’s head. The front body 1002 is configured for placement in front of eyes of a user in a reliable and comfortable manner. A display system 1080 may be disposed in the front body 1002 for presenting AR/VR imagery to the user. Sides 1006 of the front body 1002 may be opaque or transparent.

In some embodiments, the front body 1002 includes locators 1008 and an inertial measurement unit (IMU) 1010 for tracking acceleration of the HMD 1000, and position sensors 1012 for tracking position of the HMD 1000. The IMU 1010 is an electronic device that generates data indicating a position of the HMD 1000 based on measurement signals received from one or more of position sensors 1012, which generate one or more measurement signals in response to motion of the HMD 1000. Examples of position sensors 1012 include: one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU 1010, or some combination thereof. The position sensors 1012 may be located external to the IMU 1010, internal to the IMU 1010, or some combination thereof.

The locators 1008 are traced by an external imaging device of a virtual reality system, such that the virtual reality system can track the location and orientation of the entire HMD 1000. Information generated by the IMU 1010 and the position sensors 1012 may be compared with the position and orientation obtained by tracking the locators 1008, for improved tracking accuracy of position and orientation of the HMD 1000. Accurate position and orientation is important for presenting appropriate virtual scenery to the user as the latter moves and turns in 3D space.

The HMD 1000 may further include a depth camera assembly (DCA) 1011, which captures data describing depth information of a local area surrounding some or all of the HMD 1000. The depth information may be compared with the information from the IMU 1010, for better accuracy of determination of position and orientation of the HMD 1000 in 3D space.

The HMD 1000 may further include an eye tracking system 1014 for determining orientation and position of user’s eyes in real time. The obtained position and orientation of the eyes also allows the HMD 1000 to determine the gaze direction of the user and to adjust the image generated by the display system 1080 accordingly. The determined gaze direction and vergence angle may be used to adjust the display system 1080 to reduce the vergence-accommodation conflict. The direction and vergence may also be used for displays’ exit pupil steering as disclosed herein. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided including e.g. a set of small speakers built into the front body 1002.

Embodiments of the present disclosure may include, or be implemented in conjunction with, an artificial reality system. An artificial reality system adjusts sensory information about outside world obtained through the senses such as visual information, audio, touch (somatosensation) information, acceleration, balance, etc., in some manner before presentation to a user. By way of non-limiting examples, artificial reality may include virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include entirely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, somatic or haptic feedback, or some combination thereof. Any of this content may be presented in a single channel or in multiple channels, such as in a stereo video that produces a three-dimensional effect to the viewer. Furthermore, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in artificial reality and/or are otherwise used in (e.g., perform activities in) artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a wearable display such as an HMD connected to a host computer system, a standalone HMD, a near-eye display having a form factor of eyeglasses, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein. 

What is claimed is:
 1. A pupil-replicating lightguide for expanding image light, the pupil-replicating lightguide comprising: a slab of transparent material for guiding the image light therein by a series of internal reflections from opposed surfaces of the slab; and an out-coupling grating structure supported by the slab for out-coupling portions of the image light from the slab, wherein the portions are laterally offset from one another along a path of the image light in the slab, and wherein the out-coupling grating structure has a switchable distribution of out-coupling efficiency.
 2. The pupil-replicating lightguide of claim 1, wherein the out-coupling grating structure comprises a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switchable between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold.
 3. The pupil-replicating lightguide of claim 2, wherein the second threshold is at least ten times lower than the first threshold.
 4. The pupil-replicating lightguide of claim 2, wherein the plurality of switchable diffraction gratings comprises switchable gratings with spatially varying slant angle of the grating fringes along the path of the image light.
 5. The pupil-replicating lightguide of claim 4, wherein the switchable gratings are disposed in a stack configuration parallel to the opposed surfaces of the slab.
 6. The pupil-replicating lightguide of claim 2, wherein the plurality of diffraction gratings comprises a polarization volume hologram (PVH) grating.
 7. The pupil-replicating lightguide of claim 2, wherein the plurality of diffraction gratings comprises a tunable liquid crystal (LC) surface-relief grating.
 8. The pupil-replicating lightguide of claim 1, wherein the out-coupling grating structure comprises a fluidic grating.
 9. The pupil-replicating lightguide of claim 1, wherein the out-coupling grating structure comprises a grating having a wavelength-dependent refractive index contrast.
 10. A near-eye display comprising: a projector for providing image light; and a pupil-replicating lightguide coupled to the projector and comprising: a slab of transparent material for guiding the image light therein by a series of internal reflections from opposed surfaces of the slab; and an out-coupling grating structure supported by the slab for out-coupling portions of the image light from the slab, wherein the portions are laterally offset from one another along a path of the image light in the slab, wherein the out-coupling grating structure comprises a plurality of grating fringes having a switchable distribution of out-coupling efficiency.
 11. The near-eye display of claim 10, further comprising: an eye tracker for determining a position of a pupil of a user’s eye at an eyebox of the near-eye display; and a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to: cause the eye tracker to determine the position of the pupil; and, responsive to the determined position of the pupil, switch the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position.
 12. The near-eye display of claim 10, wherein the out-coupling grating structure comprises a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switchable between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold.
 13. The near-eye display of claim 12, wherein the plurality of switchable diffraction gratings comprises switchable gratings with spatially varying slant angle of the grating fringes along the path of the image light.
 14. The near-eye display of claim 12, wherein the second threshold is at least ten times lower than the first threshold.
 15. The near-eye display of claim 12, wherein the plurality of diffraction gratings comprises at least one of: a polarization volume hologram (PVH) grating; a fluidic grating; or a grating having a wavelength-dependent refractive index contrast.
 16. The near-eye display of claim 12, further comprising: an eye tracker for determining a position of a pupil of the user’s eye at an eyebox of the near-eye display; and a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to cause the eye tracker to determine the position of the pupil; and, responsive to the determined position of the pupil, switch a diffraction grating of the plurality of diffraction gratings to the high-efficiency state.
 17. The near-eye display of claim 16, wherein the controller is further configured to switch the remaining diffraction gratings of the plurality of diffraction gratings to the low-efficiency state.
 18. A method for displaying an image, the method comprising: providing image light to a pupil-replicating lightguide comprising a slab of transparent material; guiding the image light in the slab by a series of internal reflections from opposed surfaces of a slab of transparent material; out-coupling portions of the image light from the slab by an out-coupling grating structure, wherein the portions are laterally offset from one another along a path of the image light in the slab; and switching angular distribution of out-coupling efficiency of a plurality of grating fringes of the out-coupling grating structure.
 19. The method of claim 18, wherein the switching of the angular distribution of the out-coupling efficiency comprises switching a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switched between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold.
 20. The method of claim 18, further comprising: using an eye tracker to determine a position of a pupil of a user’s eye at an eyebox of the near-eye display; and, responsive to the determined position of the pupil, switching the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position. 